<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>  </title>
    <style>
        .box{
            width: 300px;
            height: 300px;
            background-color: #999;
            margin:0 auto;
            box-shadow: 0 0 100px red ;

        }
    </style>
</head>
<body>
    <div class="box"></div>
</body>
</html>